In medicine it is often desirable to monitor blood pressure, for example, over the course of an invasive procedure. Blood pressure is most reliably measured using an arterial line. The fitting of an arterial line is invasive and is most commonly performed under anaesthetic.
Ways and means of measuring blood pressure non-invasively are also known. The most common method of measuring blood pressure non-invasively is via an inflatable cuff placed around the upper arm. Whilst accurate, these measurements obstruct blood flow to the limb and therefore may only be taken intermittently.
Alternative methods of measuring blood pressure that maintain the blood flow through the interrogated artery are known. The “vascular unloading technique,” as described in U.S. patents U.S. Pat. No. 6,669,648, U.S. Pat. No. 7,390,301 and U.S. Pat. No. 8,114,025 and US patent applications US 2011/0105917 and US 2011/0105918, continuously measures the blood pressure through an inflatable cuff fitted to a finger. This known method allows blood flow to be maintained through the interrogated artery thus making long term continuous measurement possible. The pressure load on and blood congestion in the interrogated artery is limited by providing a second finger cuff on a second finger and alternating between the two cuffs when taking blood pressure measurements.
It will be appreciated that it may not be possible to directly compare blood pressure measurements taken with different measurement techniques or on different points of the body, not least due to the difference in blood pressure between the smaller arteries, for example in a finger, and the major arteries. For this reason it may be beneficial to calibrate blood pressure measurements.